|Publication number||US7123507 B2|
|Application number||US 10/922,298|
|Publication date||Oct 17, 2006|
|Filing date||Aug 19, 2004|
|Priority date||Aug 19, 2004|
|Also published as||US20060039184|
|Publication number||10922298, 922298, US 7123507 B2, US 7123507B2, US-B2-7123507, US7123507 B2, US7123507B2|
|Inventors||David W. Abraham, Yu Lu|
|Original Assignee||International Business Machines Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (6), Classifications (6), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The invention disclosed broadly relates to the field of Magnetic Random Access Memory (MRAM) and more particularly relates to the field of using magnets to assist in writing to MRAM.
Magnetic storage media are well known. Media for hard disk drives and MRAM are among the most common forms of magnetic storage media. Referring to
Also shown is MRAM memory cell 9 that including a magnetic tunnel junction (MTJ). The MTJ comprises a magnetically changeable or free region, and a proximate magnetic reference region, arranged into the MTJ device. The principle underlying storage of data in such cells is the ability to change the relative orientation of the magnetization of the free and reference regions by changing the direction of magnetization along the easy axis (“EA”) of the free region, and the ability to thereafter read this relative orientation difference. The memory cell 9 is located at each crossing point of the word lines and bit lines in the intersection region vertically spaced between the lines.
The memory cell 9 is formed on and in contact with a word line 2. The MTJ of memory cell 9 is fabricated to have a preferred axis for the direction of magnetization called the easy axis. There are two possible directions of magnetization of the memory cell 9 along this easy axis, which defines the two states of the memory cell. The desired easy axis for the memory cell 9 is set by some combination of intrinsic anisotropy, strain-induced anisotropy and shape anisotropy of the MTJ.
An MRAM cell is written by reversing the free region magnetization using applied bi-directional electrical and resultant magnetic stimuli via its respective bitline and wordline, and is later read by measuring the resultant tunneling resistance between the bitline and wordline, which assumes one of two values depending on the relative orientation of the magnetization of the free region with respect to the reference region.
The term reference region is used broadly herein to denote any type of region which, in cooperation with the free or changeable region, results in a detectable state of the device as a whole. If the free region is modeled as a simple elemental magnet having a direction of magnetization which is free to rotate but with a strong preference for aligning in either direction along its easy axis (+EA or −EA), and if the reference region is a similar elemental magnet but having a direction of magnetization fixed in the +EA direction, then two states (and therefore the two possible tunneling resistance values) are defined for the cell: aligned (+EA/+EA) and anti-aligned (−EA/+EA).
MRAM and media for disk drives both use magnetic bits to store information. The bit is typically a submicron piece of magnetic material. For example, in MRAM the free layer of the tunnel junction is the magnetic bit; it can be an ellipse 300 nm×600 nm in area and 5 nm thick. The information is stored as the direction of magnetization of the bit, either pointing right or left, to store ‘1’ or ‘0’. When the bit is in a zero applied magnetic field state, its magnetization is stable pointing either right or left. The application of a magnetic field can switch it from right to left and vice versa, to write information into the bit.
Two major types of MRAM devices exist. One of these (the first type, as described above) includes a magnetic storage element with substantially non-zero magnetic moment and with anisotropy axis substantially parallel to one of the programming current direction. The threshold of transition between the two possible stable directions of the magnetic moment is fairly well described by the Stoner-Wohlfarth model. When writing such MRAM devices, the two programming currents play different roles. The programming current that is substantially parallel to the anisotropy axis (bit-line current) determines the final state of the MRAM device, while the programming current that is substantially perpendicular to the anisotropy axis (word-line current) lowers the required bit-line current but does not prefer one state versus the other.
The other type (the second type) of MRAM device includes a magnetic storage element that constitutes two magnetically coupled elements with substantially identical magnetic moment. The magnetic coupling is such that the two magnetic elements are kept substantially anti-parallel. Furthermore, the anisotropy axis of the two magnetic elements make substantially equal angle to either programming currents. The two programming currents play similar roles during the write operation but one precedes the other temporally. A net moment is created by the early programming current and guided to rotate an angle close to 180 degrees. The early and late programming currents can be exchanged due to the fundamental symmetry of the device. When the trajectory of magnetic field encloses the so called spin-flop point, the write operation will invert the initial data in the original bit, and so the popular name of toggle MRAM arises.
For both types of MRAM, the magnitude of the programming currents is relatively large (in the range of 5 to 15 mA) compared with that normally occurs in modern digital semiconductor devices. This requirement leads to large power consumption for writing MRAM devices as well as reduced device reliability and large area. Consequently, there exists a need to reduce the write currents for both types of MRAM device.
Briefly, according to an embodiment of the present invention, a Magnetic Random Access Memory (MRAM) device is disclosed. The MRAM device includes a first linear conducting element positioned along a first axis and a second linear conducting element positioned along a second axis perpendicular to the first axis. The MRAM device further includes a magnetic element positioned at the intersection of the first linear conducting element and the second linear conducting element and a permanent magnet positioned such that its magnetic field assists in writing to the magnetic element.
In another embodiment of the present invention, the MRAM device includes a first linear conducting element positioned along a first axis and a second linear conducting element positioned along a second axis perpendicular to the first axis. The MRAM device further includes two magnetically-coupled magnetic elements positioned at the intersection of the first linear conducting element and the second linear conducting element and a permanent magnet positioned such that its magnetic field assists in writing to the two magnetically-coupled magnetic elements.
Also disclosed is a method for writing to a magnetic element in a MRAM device. The method includes providing a first linear conducting element positioned along a first axis, a second linear conducting element positioned along a second axis perpendicular to the first axis, a magnetic element positioned at the intersection of the first linear conducting element and the second linear conducting element and a permanent magnet positioned such that its magnetic field assists in writing to the magnetic element. The method further includes applying a current through the first linear conducting element and the second linear conducting element so as to write to the two magnetic elements, wherein the magnetic field of the permanent magnet assists in writing to the two magnetic elements.
The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings.
The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and also the advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings. Additionally, the left-most digit of a reference number identifies the drawing in which the reference number first appears.
It should be noted that the said magnetic field component 204 from permanent magnets placed close to the MRAM chip has magnitude substantially similar to the field components generated by the programming currents 201 and 202. The said magnetic field component 204 from permanent magnets effectively offset the intrinsic astroid curve (stability boundary) from the origin of the programming field so that less programming field is required. The reduction in required programming currents reduces power consumption and improves device reliability. The programming currents are produced from a power supply measuring about 1 volt, with the currents typically measuring about 10 milliamps—resulting in a power expenditure of about 20 milliamps for one write operation. The magnetic field produced by the permanent magnets would range from about 4 to about 80 oersteds.
It should be noted that the said magnetic field component 219 from permanent magnets placed close to the MRAM chip has magnitude substantially similar to the field components generated by the programming currents 211 and 212. The said magnetic field component 219 from permanent magnets effectively offset the intrinsic spin-flop point from the origin of the programming field so that less programming field is required. The reduction in required programming currents reduces power consumption and improves device reliability.
The computer system can include a display interface 708 that forwards graphics, text, and other data from the communication infrastructure 702 (or from a frame buffer not shown) for display on the display unit 710. The computer system also includes a main memory 706, including MRAM implementing the features of the present invention (as described herein), and may also include a secondary memory 712. The secondary memory 712 may include, for example, a hard disk drive 714 and/or a removable storage drive 716, representing a floppy disk drive, a magnetic tape drive, an optical disk drive, etc. The removable storage drive 716 reads from and/or writes to a removable storage unit 718 in a manner well known to those having ordinary skill in the art. Removable storage unit 718, represents a floppy disk, a compact disc, magnetic tape, optical disk, etc. which is read by and written to by removable storage drive 716. As will be appreciated, the removable storage unit 718 includes a computer readable medium having stored therein computer software and/or data.
In alternative embodiments, the secondary memory 712 may include other similar means for allowing computer programs or other instructions to be loaded into the computer system. Such means may include, for example, a removable storage unit 722 and an interface 720. Examples of such may include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM, or PROM) and associated socket, and other removable storage units 722 and interfaces 720 which allow software and data to be transferred from the removable storage unit 722 to the computer system.
The computer system may also include a communications interface 724. Communications interface 724 allows software and data to be transferred between the computer system and external devices. Examples of communications interface 724 may include a modem, a network interface (such as an Ethernet card), a communications port, a PCMCIA slot and card, etc. Software and data transferred via communications interface 724 are in the form of signals which may be, for example, electronic, electromagnetic, optical, or other signals capable of being received by communications interface 724. These signals are provided to communications interface 724 via a communications path (i.e., channel) 726. This channel 726 carries signals and may be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, an RF link, and/or other communications channels.
In this document, the terms “computer program medium,” “computer usable medium,” and “computer readable medium” are used to generally refer to media such as main memory 706 and secondary memory 712, removable storage drive 716, a hard disk installed in hard disk drive 714, and signals. These computer program products are means for providing software to the computer system. The computer readable medium allows the computer system to read data, instructions, messages or message packets, and other computer readable information from the computer readable medium. The computer readable medium, for example, may include non-volatile memory, such as a floppy disk, ROM, flash memory, disk drive memory, a CD-ROM, and other permanent storage. It is useful, for example, for transporting information, such as data and computer instructions, between computer systems. Furthermore, the computer readable medium may comprise computer readable information in a transitory state medium such as a network link and/or a network interface, including a wired network or a wireless network, that allow a computer to read such computer readable information.
Computer programs (also called computer control logic) are stored in main memory 706 and/or secondary memory 712. Computer programs may also be received via communications interface 724. Such computer programs, when executed, enable the computer system to perform the features of the present invention as discussed herein. In particular, the computer programs, when executed, enable the processor 704 to perform the features of the computer system. Accordingly, such computer programs represent controllers of the computer system.
Although specific embodiments of the invention have been disclosed, those having ordinary skill in the art will understand that changes can be made to the specific embodiments without departing from the spirit and scope of the invention. The scope of the invention is not to be restricted, therefore, to the specific embodiments. Furthermore, it is intended that the appended claims cover any and all such applications, modifications, and embodiments within the scope of the present invention.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US6163477 *||Aug 6, 1999||Dec 19, 2000||Hewlett Packard Company||MRAM device using magnetic field bias to improve reproducibility of memory cell switching|
|US6490217 *||May 23, 2001||Dec 3, 2002||International Business Machines Corporation||Select line architecture for magnetic random access memories|
|US6509621 *||Dec 28, 2000||Jan 21, 2003||Fujitsu Limited||Magnetic random access memory capable of writing information with reduced electric current|
|US6522577||Jun 5, 2002||Feb 18, 2003||Micron Technology, Inc.||System and method for enabling chip level erasing and writing for magnetic random access memory devices|
|US6621731||May 10, 2002||Sep 16, 2003||Sony Corporation||Magnetic memory device|
|US6633498||Jun 18, 2002||Oct 14, 2003||Motorola, Inc.||Magnetoresistive random access memory with reduced switching field|
|U.S. Classification||365/158, 365/55, 365/157|
|Aug 19, 2004||AS||Assignment|
Owner name: INTERNATIONAL BUSINESS MACHINES CORPORATION, NEW Y
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:ABRAHAM, DAVID W.;LU, YU;REEL/FRAME:015709/0400
Effective date: 20040819
|Apr 16, 2010||FPAY||Fee payment|
Year of fee payment: 4
|May 30, 2014||REMI||Maintenance fee reminder mailed|
|Jul 11, 2014||SULP||Surcharge for late payment|
Year of fee payment: 7
|Jul 11, 2014||FPAY||Fee payment|
Year of fee payment: 8